A separator disclosed in, for example, Patent Literature 1 is a decanter type centrifugal separator that performs a separation operation by applying a centrifugal force to a solution to be processed. In the centrifugal separator, a cylindrical bowl into which the solution to be processed is supplied is disposed inside a casing, and a drive mechanism (e.g., drive motor) for rotating the bowl is disposed outside the casing. A shaft serving as a rotary shaft of the bowl is rotatably supported by a bearing mechanism that is disposed so as to block an upper opening of the casing, and the power from the drive motor is transmitted to an end portion of the shaft extending to the outside of the casing. Also, a screw conveyor is disposed inside the bowl. The power from the drive motor is transmitted to the screw conveyor via a gear box serving as a differential rate generator, and accordingly the screw conveyor rotates at a rate (differential rate) having a difference relative to that of the bowl, thereby conveying a solid to an outlet.
The bearing mechanism for the shaft serving as the rotary shaft of the bowl includes a bearing that rotatably supports the shaft so that the shaft is rotatable, a housing that holds the bearing, and a mechanical seal that seals (shaft seal) a clearance portion between the housing and the shaft. The mechanical seal prevents leakage of a substance and the like within the casing to the outside of the apparatus and deterioration of the bearing and the like due to contact with the substance and the like within the casing. Conversely, the mechanical seal also prevents foreign matter from the outside air, a lubricating oil for the bearing, and the like from entering the inside of the casing through the clearance portion.
The mechanical seal has a portion where a fixed ring and a rotary ring come into sliding contact with each other, and heat is generated in this portion. If the amount of heat generation is large, the mechanical seal deteriorates. For this reason, the centrifugal separator further includes a means for supplying, in a circulating manner, a lubricating oil as a coolant. An oil unit including an oil tank, a pump, and a temperature regulator may be used as the means for supplying the lubricating oil in a circulating manner. The lubricating oil is supplied to the mechanical seal in a circulating manner while the pressure and the flow rate are adjusted, and thus the pressure balance of the shaft seal is adjusted.
However, with respect to centrifugal separators that adopt mechanical seals, it is necessary to change the pressure-resistant design specifications of portions of the bearing mechanism other than the mechanical seal depending on the set pressure inside the casing, and therefore such centrifugal separators have been divided into, for example, products for normal-pressure use and products for high-pressure use. For example, while centrifugal separators for normal-pressure use incorporate a labyrinth seal, centrifugal separators for high-pressure use cannot ensure pressure resistance with a labyrinth seal. Thus, centrifugal separators for high-pressure use have a structure in which, for example, a clearance bushing is provided instead of the labyrinth seal, and the pressure resistance is increased by, for example, adjusting the length of the clearance bushing. Therefore, there is no compatibility of spare parts and the like between centrifugal separators for normal-pressure use and centrifugal separators for high-pressure use. Furthermore, an on-site adjustment work for adjusting the pressure balance by adjusting gaps, shapes, and the like of various components (e.g., clearance bushing and the like) constituting the bearing mechanism has been burdensome. In particular, the higher the set pressure inside the casing, the more difficult the adjustment of the pressure and the flow rate of the lubricating oil.
Some decanter type centrifugal separators are of a horizontal type in which the bowl is placed in a horizontal orientation. Decanters of horizontal centrifugal separators have a structure in which the casing is vertically divided into two portions so that maintenance or removal of the bowl can be performed, and therefore have low pressure resistance and cannot be used under high-pressure conditions. In contrast, decanters of vertical centrifugal separators can be used under high-pressure conditions because a pressure tight vessel can be used as the casing. However, it is required that the bearing mechanism also be pressure-resistant. Moreover, adjustment of the pressure balance of the shaft seal is difficult. The vertical centrifugal separators have technical problems such as these described above, which the horizontal centrifugal separators do not have.
Also, there is a problem in that when terephthalic acid is subjected to solid-liquid separation under high pressure as described in Patent Literature 4, fine terephthalic acid powder that is diffused in the casing intrudes into a sliding portion of the mechanical seal and causes the mechanical seal to deteriorate. This problem is not limited to terephthalic acid, and the same problem may arise with respect to any matter to be processed that causes fine powder to be diffused in the casing.